The purpose of this paper is to explain how various thermochemical values are computed in Gaussian. The paper documents what equations are used to calculate the quantities, but doesn’t explain them in great detail, so a basic understanding of statistical mechanics concepts, such as partition functions, is assumed. Gaussian thermochemistry output is explained, and a couple of examples, including calculating the enthalpy and Gibbs free energy for a reaction, the heat of formation of a molecule and absolute rates of reaction are worked out.